Rapid kraft pulping process for bagasse and other fibrous, non-woody lignocellulosic plant materials



Sept. 30, 1958 s. B. KNAPP ET AL 2,854,334

' RAPID KRAFT PULPING PROCESS FOR BAGASSE AND OTHER FIBROUS, NON-WOODY LIGNOCELLULOSIC PLANT MATERIALS Filed'Nov. 8, 1954 BAGASSE AND OTHER FIBROUS, NON-WOODY, LIGNOCELLULOSIC PLANT MATERIALS MIX WITH SUFFICIENT WATER, NaOH AND Nags TO FORM A MIXTURE OIIARAOTERIZED BY A LIQUOR TO DRY PLANT MATERIAL RATIO OF 2-1511, A S'ULFIDITY OF 15-35% AND A OREMIOAL' USE OF 6-20%, DRY

PLANT MATERIAL BASIS HEAT THE MIXTURE UNDER PRESSURE TO PLO-210 C. OVER A TIME 7 PERIOD OF UP TO 15 MINUTES.

MAINTAIN MIXTURE AT BIO-210 C. FOR FROM 30 SECONDS TO 5 MINUTES.

PULP PRODUCT SAMUEL B KNAPP JAMES D. WETHERN IN V EN TORS AT TY.

RAPID KRAFT PULPlNG PROCESS FOR BAGASSE AND OTHER FIBROUS, NUN-WODDY LIGNO- CELLULQSEC PLANT MATERIALS Samuel B. Knapp, Washougal, and James D. Wethern, Camas, Wash, assignors, by mesne assignments, to Hawaiian Development Company, Ltd., Honolulu, Territory of Hawaii, a corporation of Hawaii Application November 8, 1954, Serial No. 467,361 13 cums. c1. 92-9 This invention relates to a rapid process for the pulping by a modified kraft procedure of bagasse and other fibrous, non-woody, lignocellulose-containing plant materials.

When pulping bagasse by the conventional commercial kraft procedures it is common practice to cook it with sodium hydroxide, sodium sulfide, and sodiuincarbonate using these chemicals in amounts of from about 20 to 30% by weight, based on the weight of the dry raw material. The bagasse is cooked 'with the chemicals for several hours at elevated temperatureand pressure, the use of such an extended cooking period heretofore having been thought essential for the production of a pulp of commercial quality, probably because it has been standard practice when pulping wood.

Thus in typical .prior art commercial procedures the n bagasse first is comminuted, if necessary, and introduced into a pressure vessel together with water and the pulping chemicals to form a mixture of the desiredcomposition. The temperature and pressure of this mixture then-are increased gradually to the selected level over a period of from 45 to 150 minutes.

The mixture is maintained at the chosen temperature and pressure for a cooking period which may range up to several hours. It then may be discharged from the pressure vessel at the cooking'pressure. In the alternative, the pressure first may be released gradually over a gassing-off period of from 10 to 30 minutes, the temperature of the mixture during this period corresponding to the pressure in the pressure vessel, after which pressure reduction it is discharged. y

We have discovered, and it is the essence of this invention, that by carefully controlling therea'ction'eonditions, the cooking period may be reduced very materially to'an overall time of but a few minutes. i This prevents the degradation of the cellulose and "atthe 'same time produces a pulp'which is exceptionally'strong and'radily bleachable. Also, it results in a very high production rate because of the short cookingt-ime required. In fact, the process may even be carried out successfully as a continuous process wherein the digestion step is in the nature of a flash heating of the plant material in the presence of the pulping chemicals.

It is a particular feature'of theproce'ss of this invention that it leads to the production of a pulp which may be bleached to a very high level of brightness without loss of pulp strength,"usin'g an amount of bleaching agent which is but a small fraction of that conventionally employed in bleaching bagass'eand similar pulps to a lower level of brightness. Thus in a typical instance, in a conventional three-stage bleaching procedure usingchlorine, caustic and hypochlorite, a bagasse pulp having a brightness of 80% G. E. R. S. and a Mullen bursting strength of 82 is obtained. However, in the bleaching of bagasse pulp obtained by the present process, a pulp having a brightness'of 88.5 G. E. R. S. and a Mullen bursting strength of 102 may be obtained with a chlorine consump- States Patent order of 30% by weight. :pulp yield substantially and at the same time makes the pulp-stronger since the pentosans exert a beneficial influence on pulp strengthduring subsequent. papermaking procedures.

lieved that this result may be attributable to the fact that in the conventional pulping procedure the lignin separated from the cellulose, or its reaction products, are redeposited on the cellulose during the long cooking period. This ligneous deposit then resists the action of the bleaching agent subsequently employed with the resuit that a bleached product of a lower brightness level is obtained. However, in the instant procedure, the separated lignin or its derivatives are not redeposited on the cellulose during the very short cooking interval. Accordingly the resulting cellulosic pulp may be bleached me higher level of brightness. v

Inaddition, the presently described process leads to the production of a high yield of pulp which is relatively very strong compared with bagasse pulps prepared by conventional methods. This desirable result is believed attributable to the fact that the short digestion period using low concentrations of cooking agents leads 'to'the formation of a product having'a high content of pentosans. Thus, whereas a bagasse pulp produced by the conventional procedure using a high concentration of cooking agents leads to the production-of a pulp having apentosan content of about 10% by weight, often only 5% or less by weight, the present procedure leads to the production of a pulp having a pentosan content of the This obviously increases the If therefore is the general object of this invention'to provide a rapid kraft process for pulping fibrous, nonwoody, lignocellulosic plant materials, such-assugar cane 'bagasse, cereal straws, cornstalks, grasses, and the like.

It is another object of this invention to provide asirn'ple process for the kraft pulping of bagasse and other agricultural residues by subjec'ting the fibrousraw material to the action of the pulping chemicals for an extremely short period of time to produce'a pulp of commercial quality which does not require subsequent mechanical defibering. A

'It is another object of this invention to provide a kraft process of cooking bagasse rapidly and economically to produce pulps of widely varying characteristics which are suitable for conversion to boards, coarse Wrapping papers, or fine quality'unbleached or bleachedfipapers.

It is another object of this invention to provide a'rapid kraftprocess of preparing pulp from bagassean'd other plant materials Which'pulp may be bleac'hed'to an unusually high degree of brightness without loss of strength, using an amount of bleaching agent which is but a fraction of that conventionally employedto produce a pulp of a lower level of brightness.

It is still a further object of this invention to provide a kraft process of cooking lignocellulose plant materials which may be applied to such materials as straw to'pro- "duce a'pulp suitable for the manufacture of fine quality papers substantially free of specks derived from the'nodes of the straw.

Considering the foregoing in greater detail and with particular reference to the drawing comprising a flow plan of the procedure of this invention:

The presently described process may be applied to lignocellulosic plant materials derived from'a variety of sources. Thus, although it is described herein with particular reference to sugarcane bagasse, it also-is applicable to cereal straws, cornstalks, grasses and other agricultural residues derived from the processing of fibrous, non-woody lignocellulosic plants. Where bagasse is the starting material it may comprise either the whole or depithed product.

The plant material, whatever its source, may be reduced to the form of small pieces or shreds. Such a size reduction may occur during prior processing steps when the plant material is a residue from commercial operations. In other cases, however, it may be reduced in size if necessary by cutting or milling it in a separate operation in apparatus of suitable construction and design.

The comminuted plant material next is placed in a closed type digester or any other suitable pressure vessel and mixed with the aqueous digesting liquor containing the pulping chemicals, i. e., sodium hydroxide, sodium sulfide, and, optionally, sodium carbonate. These materials are used in proportions such as to give a sulfidity (Na S/Na S+NaOH) 100 of, broadly, 15-35% and, preferably, 2030%.

Also, sufficient water is employed to produce a liquor to dry plant material ratio of 2-15z1, i. e., of from 2:1 to 15:1. However, when whole bagasse is employed as the starting material, a liquor to dry bagasse ratio of 5-7:1 is preferred; when depithed bagasse is employed a liquor to dry bagasse ratio of 35:1 is preferred.

Suflicient chemicals are employed in the cooking operation to give a chemical use of from 620%, preferably, from 8l6%. By chemical use is meant the combined amount of active chemicals (Na S-I-NaOH), based on the weight of oven dry plant material to be pulped.

The pulping mixture in the digester then is heated over a rise time of up to 15 minutes, preferably from 1 second to 5 minutes, the shorter rise times being applicable in a continuous, flash operation. At the end of this rise time a cooking temperature of from 140-2l0 C., preferably from 160190 C. is reached. Also, the pressure developed rises to a broad value of from about 40 p. s. i. g. to 290 p. s. i. g., as determined by such factors as the temperature, the hydraulic pressure present in the digester, etc.

The charge is maintained under pressure at this cooking temperature for a digestion period of 30 seconds to 5 minutes broadly, preferably from 1 to 3 minutes. During this period the major proportion of the lignin is separated from the carbohydrate fraction of the lignocellulose to provide a cellulose pulp.

After the digestion period has been completed, the charge may be blown at the cooking pressure or, optionally, the pressure may be reduced to any desired level and the contents of the digester blown or dumped into the receiving vessel.

The gas-off time in any event is kept at a minimum and may be substantially instantaneous. Thus it may occur over a time period of, broadly, from 1 second to 10 minutes or, preferably, from 1 second to 3 minutes. During this time the temperature of the charge decreases to the level which substantially corresponds to the blowing pressure employed.

The pulp product discharged into the receiving vessel now is ready for further processing by conventional postpulping procedures. Thus it may be separated from the cooking liquor, washed and bleached in the usual manner, and then used in the manufacture of boards, coarse papers, or fine quality papers.

The presently described kraft process for the rapid pulping of bagasse and other lignocellulosic plant materials is further illustrated in the following examples.

EXAMPLE I Six kilograms of depithed bagasse (oven dry basis) was charged into a 2.5 cubic foot stainless steel digester.

The digester was steam jacketed and had one hollow trunnion fitted so that steam or cooking liquor could be admitted directly to the charge during the cook if desired. The digester was motor driven through a gear and chain train and could be tumbled at the rate of one revolution every three minutes.

The cooking liquor was introduced into the digester in such a manner as to wilt the bagasse during the charging period. Sufiicient liquor was employed to give a chemical use of 15.5%, based on oven dry bagasse, a sulfidity of 25%, and a liquor to oven dry bagasse ratio of 4:1.

The digester was capped and its rotation initiated for agitation of the contents. The shell steam and direct steam both were turned on full. At the end of three minutes the digester was gassed-off briefly. By the end of the fourth minute a temperature of 170 C. and a pressure of p. s. i. g. had been reached.

At the end of the sixth minute the rotation of the digester was stopped and it was connected to the blow line. The blow valve was opened immediately and the cook blown to the blow tank. The total elapsed time was seven minutes, only two of which were at the cooking temperature of C.

The resultant product was a fully cooked chemical pulp. It was separated from the liquor, and washed, and was suitable for use in making paper by the conventional paper making procedure. The total yield was 64.2% of the original oven dry weight of the bagasse.

The pulp was tested by standard test procedures for bleachability, Mullen bursting strength, tensile strength (breaking length), and Schopper fold endurance, the results being compared with the values obtained in the case of a control sample of conventional kraft bagasse pulp which had been digested under the same conditions as above, except that a total cooking period of 3 hours was employed, rather than one of 7 minutes.

The pulp derived from the presently described procedure had a pentosan content of 29.8% by weight and was more easily bleached than the control, requiring substantially less bleach chemical for a given degree of bleaching. Its physical properties also were markedly superior to those of the control. The results of the tests are given below, the values having been obtained on pulp samples beaten to 200 cc., Canadian Standard Freeness.

EXAMPLE II The purpose of this example is to illustrate the superior bleaching qualities of the pulp prepared by the presently described procedure.

A quantity of pulp prepared by the procedure of Example I was bleached by a conventional three-stage bleaching procedure including chlorinating, caustic extraction, and hypochlorite treatment, using a total chlorine application of 3.83%, based on the weight of the oven-dry pulp. In the first stage the pulp at a consistency of 2% was treated with 2.68% chlorine at room temperature over a time period of 60 minutes. In the second stage the partially bleached pulp at a consistency of 12% was extracted with caustic using 1.5% by weight sodium hydroxide at 60 C. over a time period of 60 minutes. In the third stage the partially bleached and caustic extracted pulp at a consistency of 12% was treated with a solution of calcium hypochlorite containing sufficient sodium hydroxide as a buffer to maintain the pH above 8.3 throughout this entire stage. This stage was continued using 1.15% chlorine at a temperature of 34 C: over a time period of 3 hours and 45 minutes. The pulp was washed with fresh water after each stage of the bleaching treatment.

The control pulp prepared from bagasse under conventional conditions as set forth in Example I also was bleached under the same conditions as described above, but using 8% total chlorine, rather than 3.83%. The

brightness and strength properties of the two bleached products then were determined with the results given in stronger before bleaching than is thecontrol pulp, and its strength actually is enhanced by bleaching, whereas the strength of thecontrol pulp is reduced materially by that operation.

EXAMPLE In The procedure of Example I was repeated, using whole bagasse insteadof the depithed product. Also, a liquor to dry bagasse ratio of 621 was employed rather than one of 4:1 and a chemical'use of 8% instead of 15.5%.

The total yield of pulp was 75% .by weight based'on the weight of the original oven drybagasse. The pulp was suitable for use in the manufacture of corrugated board and like products despite the very low use of chemicals employed in the pulping operation.

EXAMPLE IV The procedure of Example I :again was followed, using wheat straw as the starting material rather than bagasse. The conditions used were the same with the exception that the liquor to straw ratio was 6:1, rather than 4:1.

The pulp 'product was obtained in a yield of 50% by weight based on the weight of the original oven dry straw. Itscould be bleached very easily :to a high degree of brightness.

Furthermore, the nodes of the straw stems were not completely'cooked during 'the short cooking time. As a result they could be removed easily from the pulp by a conventional screening operation.

The pulp product'obtained thus "was well suited for the manufacture of fine quality papers, the papers being substantially free of specks derived from the nodes of the straw. This is in sharp contrast to the results obtained when making paper from pulp prepared by cooking straw using the conventional kraft pulping procedures where, because of the long cooking time, the straw nodes are digested sufiiciently so that they pass through fine screens and are visible in the form of shiny specks in the final sheet of paper.

It will be apparent from a review of the data of Example I that the pulp prepared by the presently described procedure had a Mullen bursting strength which is 23% greater than that of the control. Also, in the present case the pulp had a breaking length 12% greater and a fold endurance 49% greater than that of the control.

Furthermore, it is significant to note that certain of the physical properties of the pulp produced by the presently described process after bleaching were substantially improved over those of the same pulp prior to bleaching. For example, the Mullen bursting strength, and the fold endurance of the bleached product were 3% and 42% higher, respectively, than inthe case of the unbleached product.

In contrast, the strength properties of the control pulp were influenced adversely by bleaching. Thus the Mullen bursting strength, breaking length, and fold endurance of the control pulp after bleaching were 11%,

19%, and 40% lower respectively than the same properties of the unbleached control pu lp.

Furthermore, the pulp of the present process could be bleached easily to a very high brightness level, i. c. 86.8% G. E. R. S. whereas the control pulp attained a brightness of only 79.8% G. E. R. S. in spite of the fact that the total amount of chlorine used in its bleaching was more than double that used for bleaching the pulp produced by the present procedure.

These values are all the more striking when itis considered that a high yield of pulp was obtained by a rapid pulping procedure requiring but a few minutes total elapsed time, as compared with a time of several hours required by the conventional procedure.

This results, of course, in'a very substantiallyincreased production of pulp using equipment of given capacity. Also, the chemical use is so very low as to eftect substantial additional economies.

Having thus described our invention in preferred embodiments we claim:

1. The process of pulping fibrous, non-woody, lignocellulosic plant materials which comprises forming a mixture comprising the plant material and an aqueous digesting liquor containing as active pulping agents NaOH and Na S,the mixture being characterized by a ratio of the liquor to oven dry plant material of from 2:1 to 15:1 and a sulfidity of 1535%, the combined amount of NaOH and Na s being from6 to 20% based on the weight of oven dry plant material, heating the mixture under pressure to a temperature of from 210 C. over a time period of up to 15 minutes, and maintaining the mixture under pressure at 140-210 C. for a time period of from 30 seconds to 5 minutes.

2. The process of claim 1 wherein the lignocellulosic plant material comprises sugar cane bagasse.

3. The process of claim 1 wherein the. lignocellulosic plant material comprises cereal straw. v

4. The process of claim 1 wherein the lignocellulosic plant material comprises wholesugar canebagasse and the liquor to oven dry bagasse ratio is from 5:1 to 7:1.

5. The process of claim '1 wherein the lignocellulosic plant material comprises depithed sugar cane bagasse and the liquor to oven dry bagasse ratio is from 3:1 to 5:1.

6. The process of pulping fibrous, non-woody, lignocellulosic plant materials which comprises forming 'a mixture comprising the plant material and an aqueous digesting liquor containing as active pulping agents NaOH and Na S, the mixture being characterized by a ratio of the liquor to oven dry plant material of from 2:1 to 15 :1 and a sulfidity of 1535%, the combined amount of NaOH and Na S being from 6 to 20% based on the weight of oven dry plant material, heating the mixture under pressure to a temperature of from C. over a time period of up to 15 minutes, and maintaining the mixture under pressure at 160l90 C. for a time period of from 30 seconds to 5 minutes.

7. The process of pulping fibrous, non-woody, lignocellulosic plant materials which comprises forming a mixture comprising the plant material and an aqueous digesting liquor containing as active pulping agents NaOH and Na S, the mixture being characterized by a ratio of the liquor to oven dry plant material of from 2:1 to 15 :1 and a sulfidity of 20-30%, the combined amount of NaOH and Na S being from 8 to 16% based on the weight of oven dry plant material, heating the mixture under pressure to a temperature of from 140- 210 C. over a time period of up to 15 minutes, and maintaining the mixture under pressure at 140-210 C. for a time period of from 30 seconds to 5 minutes.

8. The process of pulping fibrous, non-woody, lignocellulosic plant materials which comprises forming a mixture comprising the plant material and an aqueous digesting liquor containing as active pulping agents NaOH and Na s, the mixture being characterized by a ratio of the liquor to oven dry plant material of from 2:1 to 15:1 and a sulfidity of 1535%, the combined amount of NaOH and Na S being from 6 to 20% based on the weight of oven dry plant material, heating the mixture under pressure to 140210 C. over a time period of from 1 second to 5 minutes, and maintaining the mixture under pressure at 140 C.210 C. for a time period of from 1-3 minutes.

9. The process of pulping fibrous, non-woody, lignocellulosic plant materials which comprises forming a mixture comprising the plant material and an aqueous digesting liquor containing as active pulping agents NaOH and Na S, the mixture being characterized by a ratio of the liquor to oven dry plant material of from 2:1 to 15:1 and a sulfidity of 15-35%, the combined amount of NaOH and Na S being from 6 to 20% based on the weight of oven dry plant material, heating the mixture under pressure to 160-190 C. over a time period of from 1 second to 5 minutes, and maintaining the mixture under pressure at 160-190 C. for a time period of from 1-3 minutes.

10. The process of pulping fibrous, non-woody, lignocellulosic plant materials which comprises forming a mixture comprising the plant material and an aqueous digesting liquor containing as active pulping agents NaOH and Na S, the mixture being characterized by a ratio of the liquor to oven dry plant material of from 2:1 to 15:1 and a sulfidity of 2030%, the combined amount of NaOH and Na S being from 8 to 16% based on the weight of oven dry plant material, heating the mixture under pressure to 1402l0 C. over a time period of from 1 second to 5 minutes, and maintaining the mixture under pressure at 140-210 C. for a time period of from 1-3 minutes.

11. The process of pulping fibrous, non-woody, ligno cellulosic plant materials which comprises forming a mixture comprising the plant material and an aqueous digesting liquor containing as active pulping agents NaOH and Na s, the mixture being characterized by a ratio of the liquor to oven dry plant material of from 2:1 to 15:1 and a sulfidity of 1535%, the combined amount of NaOH and Na S being from 6 to 20% based on the weight of oven dry plant material, heating the mixture under pressure to l40-2l0 C. over a time period of up to 15 minutes, maintaining the mixture under pressure at 140-210" C. for a time period of from 30 seconds to 5 minutes, and reducing the pressure to blowing pressure over a time period of from 1 second to 10 minutes.

12. The process of pulping fibrous, non-woody, lignocellulosic plant materials which comprises forming a mixture comprising the plant material and an aqueous digesting liquor containing as active pulping agents NaOH and Na S, the mixture being characterized by a ratio of the liquor to oven dry plant material of from 2:1 to 15:1 and a sulfidity of 1535%, the combined amount of NaOH and Na s being from 6 to 20% based on the weight of oven dry plant material, heating the mixture under pressure to 140-2l0 C. over a time period of from 1 second to 5 minutes, maintaining the mixture under pressure at 140210 C. for a time period of from 30 seconds to 5 minutes, and reducing the pressure to blowing pressure over a time period of from 1 second to 3 minutes.

13. The process of pulping fibrous, non-woody, lignocellulosic plant materials which comprises forming a mixture comprising the plant material and an aqueous digesting liquor containing as active pulping agents NaOH and Na S, the mixture being characterized by a ratio of the liquor to oven dry plant material of from 2:1 to 15:1 and a sulfidity of 1535%, the combined amount of NaOH and Na S being from 8 to 16% based on the weight of oven dry plant material, heating the mixture under pressure to a temperature of from l-190 C. over a time period of up to 15 minutes, and maintaining the mixture under pressure at l60-l90 C. for a time period of from 1 to 3 minutes.

References Cited in the file of this patent UNITED STATES PATENTS 296,935 Dahl Apr. 15, 1884 629,077 Hickman July 18, 1899 789,418 Drewsen May 9, 1905 1,387,441 Brown Aug. 9, 1921 1,679,441 Nanji Aug. 7, 1928 1,782,755 Williams Nov. 25, 1930 1,868,755 Dils July 26, 1932 1,991,499 Drewsen Feb. 19, 1935 2,582,054 Michon Jan. 8, 1952 2,662,012 Teunissen Dec. 8, 1953 OTHER REFERENCES Bhat: Indian Pulp and Paper, August 1953 (Reprint), pp. 1-3. (Copy in Div. 67.)

Casey: Pulp & Paper, vol. I, published by Interscience Pub. of New York, N. Y., 1952, p. 237. 

1. THE PROCESS OF PULPING FIBROUS, NON-WOODY, LIGNOCELLULOSIC PLANT MATERIALS WHICH COMPRISES FORMING A MIXTURE COMPRISING THE PLANT MATERIAL AND AN AQUEOUS DIGESTING LIQUOR CONTAINING AS ACTIVE PULPING AGENTS NAOH AND NA2S, THE MIXTURE BEING CHARACTERIZED BY A RATIO OF THE LIQUOR TO OVEN DRY PLANT MATERIAL OF FROM 2:1 TO 15:1 AND A SULFIDITY OF 15-35%, THE COMBINED AMOUNT OF NAOH AND NA2S BEING FROM 6 TO 20% BASED ON THE WEIGHT OF OVEN DRY PLANT MATERIAL, HEATING THE MIXTURE UNDER PRESSURE TO A TEMPERATURE OF FROM 140 210*C. OVER A TIME PERIOD OF UP TO 15 MINUTES, AND MAINTAINING THE MIXTURE UNDER PRESSURE AT 140-210*C. FOR A TIME PERIOD OF FROM 30 SECONDS TO 5 MINUTES. 